The Magic Behind the Volt's 230 M.P.G.

By JIM MOTAVALLI

Published: August 16, 2009

THE debate over General Motors' claim that the Chevrolet Volt will get the equivalent of 230 miles a gallon in the city has probably been good for calculator sales, as analysts pore over the numbers and offer their own predictions about the likely performance of this hybrid-electric car, which makes its debut in late 2010.

To arrive at its controversial number, Tony Posawatz, vehicle line director for the Volt, said G.M. used the Environmental Protection Agency's proposed methodology for measuring plug-in hybrid vehicles in city driving and the company's data on real-world driving.

Mr. Posawatz conceded that miles per gallon was ''probably not the best measure of goodness'' for evaluating the Volt, but said that ''it's what people are accustomed to, because m.p.g. calculations have been around since there have been vehicles.'' The public, he said, ''doesn't really understand this yet, but debate on the subject is good. The Volt is significantly different than your father's Oldsmobile.''

Mr. Posawatz said that because the Volt can travel 40 miles on a daily charge, ''theoretically many drivers will get infinite fuel economy. But even if you drive 25 miles to work, and 25 home, then an extra 10 miles elsewhere, you'll only travel 20 miles a day using gasoline.''

Any actual number crunching is highly speculative, because the E.P.A. isn't talking about its methodology. Dave Ryan, a spokesman for the agency, said in an e-mail message, ''If you're specifically asking about the procedures, they're still preliminary, so there's nothing to discuss yet.''

Cathy Milbourn, an E.P.A. spokeswoman, referred inquiries back to G.M., where Rob Peterson, a G.M. spokesman, said, ''Our performance numbers are based on their methodology.'' In an earlier statement, the E.P.A. said it ''has not tested a Chevy Volt and therefore cannot confirm the fuel economy values claimed by G.M.''